First, video it if you can.
Every good researcher videos the focus group. This is because a focus group is a psychological experiment and you will have to use your ability to read and understand human behavior. The info you gather will need to be interpreted, and having a recording of the group really, really helps.
Second, come up with good questions.
Lots of them. Then, PRIORITIZE your questions, because a good focus groups almost always runs over time. To come up with good questions, think about what you want to know in order to make decisions. So, think first of all the decisions you have to make. Then, think about what you would like to know to help you make a better decision. Then, think what you would like to know from the target audience that might provide that info.
Third, let the conversation flow.
While you need to have your questions prepared, at the same time, be ready to let the conversation flow and be more free form in order to get at the "whys" behind the answers your participants give. Again, its psychological - you want to know the rational AND, more importantly, emotional reasons people behave a certain way.
Finally, remember success relies on your skills to interpret & understand human behavior.
If you don't know how to do this, hire a professional who can. Participants will not be able to articulate the "whys" behind their behavior very often. You will have to draw it out of them with questions that will flow from the answers they give.